The present invention relates to sucker rods used in producing oil wells and more particularly, to sucker rods with high corrosion resistance and the process for manufacturing these sucker rods.
A conventional assembly for oil recovery comprises a deep well pump element placed at the bottom of the well. This deep well pump is mechanically activated by a walking beam pumping unit which is connected by one end to a power source and by the other end to a string of steel rods that interconnect themselves to form a string extended to the inside of the well, with the string connected by its other end to the deep well pump.
During pumping, the string of rods preferably performs a reciprocating or alternative movement, which may produce deflections of the string. The sucker rods are thereby subjected to wear due to frictional contact with the inner wall of the production tubing. Even though the fluid environment serves as a lubricant, abrasion does occur over the surface of the sucker rods. Additionally, tools used during assembly, such as those used for centering the string, may cause tearing of the rod surface.
In the case of hydrocarbon wells, the fluid includes dissolved salts and undissolved minerals which may have an additional abrasive effect on the rod surface.
At the same time that abrasion occurs, the metal in the sucker rods is subjected to a hard corrosive attack caused by xe2x80x9cdown-holexe2x80x9d chemicals.
Various different geographical locations of the well present various different problems with respect to chemical attack on the metal composition of the rods. The presence of hydrogen sulfide (H2S), sulfurs (HS, S), water, salty water, hydrogen ions, CO2 in aqueous solution, and other corrosive chemical compounds, finally weaken the rods structure, thereby reducing their fatigue limit. When the attack is particularly harsh, sucker rods break.
When a rod fails, the whole sucker rod string needs to be pulled from the well and inspected, and defective rods must be replaced. This procedure increases costs when it becomes frequent. Additional costs related to corrosion problems result in losses in production, added costs for new materials, and increased pulling costs.
To prevent the effects of the chemical attack, several metallic coatings were proposed to apply to the rod surface to act as a barrier between the main metal body of the rod and the deleterious chemicals in the down-hole chemicals.
In addition, the presence of different metallic materials in contact with the carbon steel rods, forms galvanic couples that may affect the corrosion kinetics of the rods.
Aluminum (99.9%) coatings were applied and it behaved well in sulfide environments. However, aluminum is anodic with regards to carbon steel of the sucker rods and exercises cathodic protection over it (sacrifice anode). This means that once the aluminum coating is pitted, the attack continues until the coating disappears, thereby its life expectancy is not particularly long. Aluminum gets pitted in neutral solutions of chlorides and pitting potential decreases as the concentration of chloride ion increases. In solutions with high chloride contents, high CO2 pressure and mildly acid pH, pitting potential is very low, being close to corrosion potential and not exhibiting re-passivation capacity.
GB Patent No. 825,152 discloses a composite article of shaft-like form comprising a steel core and cast thereupon, a casing of aluminum bronze, the steel core having a cross-sectional area of at least 50% and not greater than 75% of the total cross-sectional area of the article. The composite articles are manufactured by casting a copper base alloy containing 7-12% by weight of aluminum around a steel core component within a mould. The method disclosed by this GB patent could not be applied to perform sucker rods. Casting the aluminum bronze alloy around the steel core within a mould involves working with high temperatures (melting temperatures), at which the steel core would be subjected to a new tempered process, thereby lowering the tensile strength of the steel core. In addition, the large equipment for manufacturing sucker rods, which are about 7 meters long, makes the process technologically impractical. Moreover, the thickness of the casing in GB patentxe2x80x94from 25 to 50% of the cross-sectional area- makes this procedure very expensive.
U.S. Pat. No. 4,045,591, provides a method to treat a sucker rod, which comprises shot peening the exterior surface of the rod and coating the exterior surface by spraying a stainless steel metallic alloy using an electrical arc metal spray apparatus.
Stainless steel alloys, such as 13% chromium steels and 18% chromium steels, provide a good option against carbonic corrosion and exhibit a low tendency to localized corrosion, but are cathodic coatings, thereby nobler than carbon steel base material. Thus, in the event the base would be exposed, there might be a harsh attack against base material. In general, stainless steel coatings crack when subjected to fatigue (traction and compression), bending and/or handling damages, thereby causing the base material to become exposed. Said exposure activates the galvanic couple, thus starting a harsh attack against base material.
While the prior art discloses a wide variety of methods for protecting sucker rods, no teaching has been found for a sucker rod material that will interact beneficially with the corrosive environment. All the efforts have been drawn to preventing the action of the corrosive environment using inert coatings.
Thus, it is desirable to provide a sucker rod capable of resisting corrosion even under severe conditionsxe2x80x94fatigue, bending and/or blowsxe2x80x94like those found at hydrocarbon production wells.
It has now been unexpectedly found that providing a sucker rod coated with a copper base alloy protects the metal core of the rod against the corrosive environment as well as regenerates itself in order to coat and protect its damaged areas.
It is an object of the present invention to provide a sucker rod coated by a metal alloy that is capable of recovering affected areas in order to protect the metal core of the rod against corrosion.
The present invention provides a new sucker rod with high resistance to corrosion, comprising a carbon steel core, which can be either alloyed or not, whose surface is coated by a metallic alloy, wherein said alloy is a copper base alloy.
Preferably, copper base alloy comprises copper in a 50 to 99.9% by weight.
A preferred copper base alloy comprises between 87 and 96% by weight of Cu and between 3 and 12% by weight of Al.
Still further preferred is a sucker rod coated by a copper base alloy comprising 90% by weight of copper, 5% by weight of aluminum and 0.5% by weight of iron, identified as aluminum bronze.
Other suitable alloys that can be applied as coatings are those comprising between 55 and 65% by weight of Cu, between 15 and 20% by weight of Ni and between 17 and 27% by weight of Zn.
Yet another suitable copper base alloy comprises 76% by weight of Cu, 22% by weight of Zn and 2% by weight of Al.
Still another object of this invention is to provide a process of manufacturing sucker rods with high resistance to corrosion, comprising the stages of:
1. cleaning the surface of the rods to remove oil contaminants;
2. keeping the surface free of dust or other environmental contaminants;
3. grit blasting the surface with stell particles
4. applying a copper base alloy over the surface.
The new rods of the present invention present the unexpected advantage of longer life when applied down-hole, particularly in a hydrocarbon well, wherein the aqueous environment is a saline solution containing H2S, CO2, S= ions and SO4= ions.
The applied coating of the present invention can passivate through the formation of a self-protecting CuSO4 and/or CuS layer, into a damaged surface. This process would take place due to reaction with the surrounding environment, which is rich in sulfate and/or sulfide ions.
The aluminum copper coating used in the present invention behaves cathodically against the carbon steel of the base. However, in such an environment, the immediate formation of a copper salt layer over the whole surface, even over a porous or damaged surface, shall render a corrosion potential not far from the one corresponding to the coating, thereby blocking the galvanic coupling activation mechanism and consequently stopping the selective dissolution of iron from the base material.
Advantageously, contrary to stainless steel coatings, the coating used in the present invention presents more flow than the base material, thereby reducing cracking possibilities and the exposure of the base material.